The present invention relates to a display device, and more particularly, to a 3D display device including a liquid crystal lens having a lens function on the display surface side of a liquid crystal display panel.
A display device that can switch between three-dimensional (3D) display and two-dimensional (2D) display with naked eyes without glasses includes, for example, a first liquid crystal display panel for performing image display, and a second liquid crystal display panel provided on the display surface side (observer side) of the first liquid crystal panel to form a parallax barrier that allows light to be separately incident on the right and left eyes of the observer in 3D display. In such a display device that can switch between 2D display and 3D display, the refractive index in the second liquid crystal display panel is changed by controlling the alignment of the liquid crystal molecules in the second liquid crystal display panel, to form lens (lenticular lens, cylindrical lens array) areas extending in the vertical direction of the display surface and arranged side by side in the lateral direction, in order to direct the light of the pixels corresponding to the left and right eyes into the observer's eye.
With respect to the 3D display device of the liquid crystal lens type having such a structure, for example, an auto-stereoscopic display device is described in Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2009-520231. In the display device described in Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2009-520231, a planar electrode is formed on one of two transparent substrates facing each other with a liquid crystal layer interposed therebetween. Then, a strip-like electrode (linear electrode) extending in the lens formation direction is formed on the other transparent substrate. The linear electrode is arranged side by side in the lens arrangement direction. With this configuration, the switching control between 2D display and 3D display can be achieved by adjusting the refractive index of liquid crystal molecules by controlling the voltage applied to the strip-like electrode and the voltage applied to the planar electrode. Further, the liquid crystal lens described in Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2009-520231 is a liquid crystal lens of TN alignment.
Japanese Patent No. 2862462 descries a structure in which an optical characteristic variable lens is provided between the electrodes on the liquid crystal display panel, to form a 3D image by controlling the lens characteristics by applying a voltage to the electrodes between which the optical characteristic variable lens is placed.
FIG. 10 is a cross-sectional view of the structure of a conventional liquid crystal lens. In FIG. 10, a first electrode 11 is formed in a planar solid-state inside a first substrate 10 which is a transparent substrate, and a first alignment film 12 is formed on the first electrode 11. Then, a second electrode 21 having a strip-like (comb-like) shape is formed inside a second substrate 20 which is a transparent substrate. A second alignment film 22 is formed so as to cover the second electrode 21 formed in the second substrate 20. The alignment direction of the first alignment film 12 and the alignment direction of the second alignment film 22 are the same. The first substrate 10 and the second substrate 20 are preferably glass substrates, but may also be transparent plastic substrates. A liquid crystal layer 60 is provided between the first substrate 10 and the second substrate 20.
The electrode width of the comb electrode formed in the second substrate 20 is w2, the pitch between the comb electrodes is Q, and the comb electrode interval is s. The distance between the first substrate 10 and the second substrate 20, namely, the thickness of the liquid crystal layer is d. The liquid crystal has positive dielectric constant anisotropy. In a 3D image display device using a liquid crystal lens, it is possible to display a 3D image by applying a voltage between the first electrode 11 and the second electrode 21, and to display a 2D image when no voltage is applied between the first electrode 11 and the second electrode 21.
FIG. 11 is a cross-sectional view showing the principle of the 3D image formation using a liquid crystal lens. In FIG. 11, human eyes view the image formed on the display device through the liquid crystal lens. In FIG. 11, the image for the right eye is R, and the image for the left eye is L. In FIG. 11, the pitch of a liquid crystal lens 100 is Q, and the pixel pitch of a display device 200 is P. Further, the distance between the centers of the human left and right eyes, namely, the interocular distance is B. In general, the interocular distance B is assumed to be 65 mm. The relationship between the pitch Q of the liquid crystal lens and the pixel pitch P of the display device is as shown in the equation 1.
                    Equation        ⁢                                  ⁢        1                                                                                ⁢          1                ⁢                                  ⁢                  Q          =                                    2              ⁢              P                                      (                              1                +                                  P                  /                  B                                            )                                                          (        1        )            
FIG. 12 is a schematic cross-sectional view of the 3D image display device using the liquid crystal lens 100 to which the present invention is directed. In FIG. 12, the liquid crystal lens 100 and the display device 200 are bonded with an adhesive 300. The adhesive 300 is transparent and, for example, a UV (ultraviolet) curing resin is used. A liquid crystal display device or an organic EL display device is used for the display device 200.
FIGS. 13A and 13B are plan views of the liquid crystal display lens corresponding to B-B′ in FIG. 12. In FIGS. 13A and 13B, the entire display area of the first substrate 10 is covered by the first electrode 11. The second electrode 21 having a comb-like shape is formed in the second substrate 20. The second electrode 21 is connected by a bus electrode at an end thereof. Here, FIG. 10 is a cross-sectional view corresponding to the A-A′ cross section in FIGS. 13A and 13B.
FIGS. 14A, 14B, and 14C are cross-sectional views showing the principle of the liquid crystal lens. When a voltage is applied between the first electrode 11 and the second electrode 21, electric lines of force F are generated as shown in FIG. 14A. If no voltage is applied between the first electrode 11 and the second electrode 21, the liquid crystal is horizontally aligned as shown in FIG. 14B. Note that in the drawings of the present application, the pretilt angle is ignored to avoid complications.
When a voltage is applied between the first electrode 11 and the second electrode 21, liquid crystal molecules 61 above the second electrode 21 rise up, and are horizontally aligned between the comb electrodes as shown in FIG. 14C. This results in a distribution in the refractive index, and a gradient index (GRIN) lens is formed.
A conventional common liquid crystal lens is shown in FIGS. 10 to 14C. In the liquid crystal lens having such a structure, disclination appears above the comb electrodes. Thus, there is a problem that crosstalk increases as the incident light is scattered in the upper part of the electrodes. Here, disclination is a discontinuous line due to the alignment of liquid crystal molecules, and crosstalk is a phenomenon in which the left eye image and the right eye image are not sufficiently separated. If the crosstalk is large, the 3D image is seen just as a double image.
On the other hand, as shown in FIGS. 15A and 15B, the alignment of the liquid crystal molecules in the liquid crystal lens is converted into TN alignment. In this case, if a polarizing plate 13 is provided on the side opposite to the side of the liquid crystal of the second substrate 20, the crosstalk due to the disclination may be reduced. At this time, TN is 90-degree twisted alignment. In other words, in FIG. 15A, the alignment direction of the first alignment film (not shown) formed in the first substrate 10, and the alignment direction of the second alignment film (not shown) formed in the second substrate 20 are 90 degrees. The mechanism will be described below.
FIG. 15A shows the state in which no voltage is applied between the first electrode 11 and the second electrode 21. At this time, the image from the display device is not affected by the liquid crystal lens. FIG. 15B shows the state in which a voltage is applied between the first electrode 11 and the second electrode 21. The liquid crystal molecules are aligned so that a lens is formed between the comb electrodes which are the second electrodes 21. Meanwhile, the electric lines of force F are directed in the perpendicular direction to the second electrode 21, so that the liquid crystal molecules 61 are also perpendicularly aligned. In other words, the light from the display device does not transmit in this portion. As a result, it is possible to prevent the crosstalk.
In FIGS. 15A and 15B, it is desirable that the transmission axis of the polarizing plate 13 is tilted approximately 90 degrees with respect to the polarization direction of the light output from the display device. If the display device is a liquid crystal display device, the output light is polarized light. However, if the display device is an organic EL display device, it is necessary to attach the polarizing plate on the surface of the organic EL display device.
FIG. 16 is a cross-sectional view showing the details of this state. FIG. 16 is a cross-sectional view showing the relationship between the polarization direction of the incident light and the polarization direction of the output light, with respect to the transmission axis of the first polarizing plate 13, when no voltage is applied between the first electrode 11 and the second electrode 21. In FIG. 16, when the liquid crystal lens has TN alignment in the initial alignment, the incident polarized light is rotated at an angle of 90 degrees within the liquid crystal layer when no voltage is applied. Thus, if the input polarization direction is the X axis direction, the output polarization direction is the Y axis direction. The incident light is transmitted if a polarization transmission axis PA of the first polarizing plate is in the Y axis direction. In the 2D display in which no voltage is applied between the first electrode 11 and the second electrode 21, the liquid crystal lens has no influence on the output light from the display device.
On the other hand, when a voltage is applied to the liquid crystal lens of TN alignment, the alignment of the liquid crystal molecules 61 is as shown in FIG. 15B. As can be seen in FIG. 15B, the liquid crystal molecules 61 rise up above the second electrode 21, so that the optical rotation is lost. However, in the vicinity of the center between the second electrodes 21 which are the comb electrodes, the alignment of the liquid crystal molecules 61 is not substantially changed from the initial alignment. As a result, optical rotation occurs and the incident light polarization axis is rotated by 90 degrees. Thus, although the light is shielded above the second electrode 21, the light transmits between the second electrodes 21. The conventional liquid crystal lens has had a problem that disclination appears above the second electrode 21, causing crosstalk to increase due to the scattering of the light. However, this problem may be solved by the configuration shown in FIGS. 15A and 15B.
Thus, a liquid crystal lens of TN alignment was formed by the following parameters:    Liquid crystal physical property . . . Δn=0.2    Liquid crystal gap d: 30 μm    Panel size: 3.2″    Number of pixels . . . 480×854    Pixel pitch P: 79.5 μm    Lens pitch Q: 158.8058 μm    Electrode width: 10 μm
However, in the liquid crystal lens described above, the ratio between the liquid crystal gap d1 and the electrode width w2, (d/w2), is as large as 3, so that the electric field is extended in the substrate in-plane direction. From this it is found that sufficient vertical electric filed is not generated. As a result, the light-shielding effect may not be obtained sufficiently above the second electrode 21.
FIGS. 18A and 18B are an example of the transmission distribution of the liquid crystal lens of TN alignment. In FIGS. 18A and 18B, the horizontal axis is the position and the vertical axis is the transmission. In the ideal transmission distribution shown in FIG. 18A, the transmission is approximately zero in the vicinity of the second electrode 21. However, in the actual sample, the transmission is not sufficiently reduced in the vicinity of the second electrode 21 as shown in FIG. 18B, for the reasons described above. In other words, the desired light-shielding effect may not be obtained.
In a common TN type liquid crystal display device, the liquid crystal gap is about 4 μm, while the electrode width is several tens to hundreds of μm. In other words, the ratio between the gap and the electrode width is very small.